The development of drug resistance by malignant cells is a common clinical problem. The appearance of a single viable drug-resistant mutant cell could lead to treatment failure. The need to understand the genetic mechanisms of drug resistance arises from the possibility that innovative treatment strategies using currently available clinical agents might circumvent or delay the development of resistance. Also, a more complete understanding of the genetic, molecular, and cellular mechanisms involved in drug resistance may suggest new agents or drug combinations specifically lethal to resistant cells. This application proposes to identify the gene responsible for conferring murine leukemia P388 cells resistant to vincristine and adriamycin. The general approach will be to create a drug-resistant cell line of a species not of mouse origin by transfection using DNA obtained from the drug-resistant murine cells. A library of genomic clones will be constructed from the transfected, drug-resistant cells, and this library will be screened by hybridization to identify the clones containing mouse DNA sequences. The ability of the hybridizing clones to confer drug resistance will be verified by transfection assays, and the minimum sequence from the clones sufficient for the transfer of the resistance phenotype will be determined. The gene structure and product will be determined by comparison with a cDNA clone and nucleotide sequencing. Finally, the expression of the gene in normal mouse, mouse leukemia, and drug-resistant mouse leukemia cells will be studied.